Download 4. Motion, Energy, and Gravity

Making Sense of the Universe:
Understanding Motion, Energy, and Gravity
© 2010 Pearson Education, Inc.
How do we describe motion?
Precise definitions to describe motion:
• Speed: Rate at which object moves
ö
speed = distance æçèunits of m
÷
s
ø
time
Example: 10 m/s
• Velocity: Speed and direction
Example: 10 m/s, due east
• Acceleration: Any change in
velocity units of speed/time (m/s2)
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The Acceleration of Gravity
• All falling objects
accelerate at the
same rate (not
counting friction
of air resistance).
• On Earth, g ≈ 10
m/s2: speed
increases 10 m/s
with each second
of falling.
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The Acceleration of Gravity (g)
• Galileo showed that
g is the same for all
falling objects,
regardless of their
mass.
Apollo 15 demonstration:
https://www.youtube.com/watch?v=KDp1tiUsZw8
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Momentum and Force
• Momentum = mass  velocity
• A net force changes momentum, which
generally means an acceleration (change in
velocity).
• Rotational momentum of a spinning or orbiting
object is known as angular momentum.
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How is mass different from weight?
• Mass – the amount of matter in an object
• Weight – the force that acts upon an object
You are weightless in free-fall!
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Why are astronauts weightless
in space?
• There is gravity in space.
• Weightlessness is due to a
constant state of free-fall.
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How did Newton change our
view of the universe?
• Realized the same physical laws
that operate on Earth also
operate in the heavens
 one universe
• Discovered laws of motion and
gravity
• Much more: experiments with
light, first reflecting telescope,
calculus…
Sir Isaac Newton
(1642–1727)
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What are Newton’s three laws of
motion?
Newton’s first law of
motion: An object moves at
constant velocity unless a net
force acts to change its speed
or direction.
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Newton’s second law of motion:
Force = mass  acceleration
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Newton’s third law of motion:
For every force, there is always an equal and opposite
reaction force.
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Why do objects move at constant
velocity if no force acts on them?
Objects continue at constant velocity because of
conservation of momentum.
• The total momentum
of interacting objects
cannot change unless
an external force is
acting on them.
• Interacting objects
exchange momentum
through equal and
opposite forces.
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What keeps a planet rotating and
orbiting the Sun?
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Conservation of Angular
Momentum
Angular momentum = mass x velocity x radius
• The angular momentum of an object cannot change
unless an external twisting force (torque) is acting
on it.
• Earth experiences no twisting force as it orbits the
Sun, so its rotation and orbit will continue
indefinitely.
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Angular momentum conservation also explains why objects
rotate faster as they shrink in radius.
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Where do objects get their energy?
• Energy makes matter move.
• Energy is conserved, but it can:
– transfer from one object to another
– change in form
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Basic Types of Energy
• Kinetic (motion)
• Radiative (light)
• Potential (stored)
Energy can change type,
but cannot be created or
destroyed.
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Thermal Energy:
the collective kinetic energy of many particles
(for example, in a rock, in air, in water)
Thermal energy is related to temperature but it is NOT
the same.
Temperature is the average kinetic energy of the many
particles in a substance.
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Temperature Scales
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Thermal energy is a measure of the total kinetic energy of all
the particles in a substance. It therefore depends on both
temperature AND density.
Example:
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Gravitational Potential Energy
• On Earth, depends on:
– object’s mass (m)
– strength of gravity (g)
– distance object could
potentially fall
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Gravitational Potential Energy
• In space, an object or gas cloud has more gravitational
energy when it is spread out than when it contracts.
A contracting cloud converts gravitational potential
energy to thermal energy.
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Mass-Energy
Mass itself is a form of potential energy:
E =
• A small amount of mass can
release a great deal of energy (for
example, an H-bomb).
• Concentrated energy can
spontaneously turn into particles
(for example, in particle
accelerators).
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2
mc
Conservation of Energy
• Energy can be neither created nor destroyed.
• It can change form or be exchanged between
objects.
• The total energy content of the universe was
determined in the Big Bang and remains the
same today.
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What determines the strength of gravity?
The universal law of gravitation:
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers.
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How does Newton’s law of gravity extend
Kepler’s laws?
• Kepler’s first two laws apply to all orbiting
objects, not just planets.
• Ellipses are not the only
orbital paths. Orbits can be:
– bound (ellipses)
– unbound
• parabola
• hyperbola
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Center of Mass
• Because of momentum
conservation, orbiting
objects orbit around
their center of mass.
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Newton and Kepler’s Third Law
Newton’s laws of gravity and motion showed
that the relationship between the orbital period
and average orbital distance of a system tells us
the total mass of the system.
Examples:
• Earth’s orbital period (1 year) and average distance (1 AU)
tell us the Sun’s mass.
• Orbital period and distance of a satellite from Earth tell us
Earth’s mass.
• Orbital period and distance of a moon of Jupiter tell us
Jupiter’s mass.
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Newton’s Version of Kepler’s Third
Law
p2 =
4p 2
G(M1 + M 2 )
a3
OR
2
4p
M1 + M 2 =
G
p = orbital period
a = average orbital distance (between centers)
(M1 + M2) = sum of object masses
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3
a
2
p
How do gravity and energy together
allow us to understand orbits?
• Total orbital energy
(gravitational +
kinetic) stays
constant if there is
no external force.
• Orbits cannot
change
spontaneously.
Total orbital energy stays constant.
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Escape Velocity
• If an object gains enough
orbital energy, it may
escape (change from a
bound to unbound orbit).
• Escape velocity from
Earth ≈ 11 km/s from sea
level (about 40,000 km/hr)
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How does gravity cause tides?
• Moon’s gravity pulls harder on near side of Earth than
on far side.
• Difference in Moon’s gravitational pull stretches Earth.
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Tides and Phases
Size of tides depends on
phase of Moon.
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Tidal Friction
• Tidal friction gradually slows Earth’s rotation (and
makes the Moon get farther from Earth).
• The Moon once orbited faster (or slower); tidal friction
caused it to “lock” in synchronous rotation.
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